The use of solar energy, through photovoltaic, thermal or thermo-photovoltaic technologies, is performed either by exposing a cell receiving directly at the incident sunlight, or concentrating the sunlight with special optical systems before directing it to the respective receiving cell.
The known optical systems may comprise reflecting surfaces and also concentration means that use lenses. Within this context, the need is felt of focusing the amount of effective light, gathered by the optical concentration system, on an active or passive receiving surface, in the most uniform possible manner.
In particular, both within the photovoltaic scope and in the thermal scope, it is preferable to avoid concentration peaks on the receiving surface which may cause thermal stresses and undesired variations in the energy transformation efficiency.
For example, in the photovoltaic scope, square or rectangular cells are used, consisting of sub-cells connected in a series that are set up for using the energy associated to the incident light within a certain wavelength range.
In fact, the efficiency of converting the solar energy into electric energy is also affected by the radiance uniformity besides the operating temperature of the cell: the more the irradiation on the cell is evenly distributed, the higher the energy conversion efficiency.
In the case of sunlight, the effective portion of the energy to be used within a photovoltaic scope in general is comprised between 300 nm and 2000 nm of the wavelength.
Multijunction photovoltaic cells must be used in order to capture all the energy, consisting of multiple layers of superimposed materials, each one sensitive to a predetermined wavelength portion.
These types of photocell can reach the theoretical limit of about 70% conversion efficiency of the solar light energy into electric energy.
Triple cells are currently used which allow obtaining conversion efficiencies even higher than 40%.
Therefore, herein and hereinafter, by effective light it is meant the light portion defined in the wavelength range to be used in order to convert the sunlight energy into electric energy. Such effective light is thus defined by the upper and lower limit values that delimit said range.
In order to obtain good operation of the photocell, for said effective light it is necessary for the radiance on the receiving surface to be as uniform as possible within a predetermined range, avoiding peaks, that is, energy concentrations having excessive density.
Such uniformity must be kept while ensuring an acceptance angle, that is, the deviation from the ideal angle with which the concentrated light beam hits the receiving surface. In the case of a lens with flat surface exposed to the sun, the ideal design incidence angle of the sun rays on the lens surface is 90°; the accepted deviation, defined as acceptance angle, is the one that in any case keeps a conversion efficiency equal to at least 90% of the maximum efficiency. Two fundamental technical aspects need to be addressed to have a good operation of the lens within the acceptance angle limits: the operating accuracy of the designed optical system (lenses, coupling mechanics) and the accuracy of the electromechanical solar tracking system.
At present, the optical systems used for concentration use small sized spherical lenses or more frequently, large sized Fresnel lenses.
These lenses produce various types of optical aberrations, in particular chromatic aberrations due to the variation of the refractive index of the lens material as the wavelength of the effective light captured varies. In particular, with refractive optics, the light with shorter wavelength undergoes a greater deviation, the crossed thickness being equal, compared to light with longer wavelength.
Moreover, spherical lenses, of the Fresnel type too, produce the so-called spherical aberration, that is, the distribution of the focuses of the light beams coming from different regions from the lens on a non accurate area.
These aberrations cause irregularities in the uniformity with which a receiving surface of a cell may be illuminated, causing efficiency losses and can in any case be managed thanks to the relatively small thicknesses of the lenses.
The above problems are quite manageable with usual technologies with systems that use small diameter (100 mm) lenses and small sized photocells; on the other hand, they become insurmountable as the lens size increases, that is, with diameters larger than 200 mm.
In fact, the lens concentration photovoltaic technology has so far used Fresnel lenses that exhibit fewer problems, but in any case introduce many aberrations.